The present invention relates generally to coiled steel strip and more particularly to coiled steel strip with a solid lubricant coating.
Coiled steel strip is the starting material for a number of manufacturing operations involving cold deforming, such as a stamping or drawing operation. These manufacturing operations are used to produce parts for automobiles and appliances, for example.
Before the steel strip undergoes the cold deforming operation, it is subjected to a blanking operation in which the coiled strip is uncoiled and cut into smaller pieces called blanks. The blanks are then individually subjected to the cold deforming operation. Incident to the blanking operation is the employment of equipment including metering rollers which meter the uncoiled strip to the blanking station and leveling or flattening rollers which remove from the steel strip any residual curvature or "coil set" carried over from the coil.
To facilitate the cold deforming operation, the surfaces of the steel strip are coated with a lubricant prior to cold deforming. It is also desirable to protect the steel strip against corrosion between the time it was coiled at the steel mill and the time it is uncoiled incident to the blanking operation. The coiled steel strip can be coated with a film of material which functions both as a corrosion resisting agent and as a lubricant during the cold deforming operation. Coiled steel strip coated with such material is known as pre-lubed strip. The coating material can be either liquid or solid. An example of a solid coating material for pre-lubed steel strip is described in Sech, U.S. Pat. No. 4,753,743, entitled "Hot Melt Metal Working Lubricant", and the disclosure thereof is incorporated herein by reference.
Many of the considerations involved in the cold deforming of steel strip, and in the selection of a pre-lube coating material for steel strip subjected to cold deforming are discussed in detail in the Sech patent and need not be repeated here. The solid lubricant coating disclosed in the Sech patent comprises, in a general sense, (a) a major portion composed of at least one substantially saturated ester formed of a polyhydric alcohol and at least one carboxylic acid; (b) a plasticizer for (a); and (c) a small amount of a polymeric composition. The Sech patent contains a number of examples in which steel blanks are coated with a solid lubricant having a coating weight in the range 100-1000 mg/ft..sup.2 (1.08-10.8 g/m.sup.2).
A problem which can arise incident to a blanking operation performed on pre-lubed steel strip is slippage of the strip at the metering rollers and the leveling rollers. Slippage at the metering or leveling rollers can cause variations in the size of the blanks which is undesirable. Slippage can occur if there is too much lubricating material on the steel strip. On the other hand, during the cold deforming operation, if there is too little lubricating material on the steel strip, the lubricity of the steel strip is impaired, and this can have an adverse effect on the cold deforming operation and on the part produced thereby.
There is a paper delivered Feb. 23, 1987, SAE Technical Paper Series, 870648, entitled "Tribological Behavior of Solid Lubricant Films on Bare and Coated Sheet Steel Products", Phillip L. Coduti, author. This paper describes tests conducted on a steel strip coated with a solid lubricant having a coating weight of 200.+-.25 mg/ft..sup.2 (2.16.+-.0.27 g/m.sup.2) on each side. One of the tests involved varying the hardness of the coating by increasing the oil content thereof. Hardness was determined with a needle penetration test: the deeper the penetration, the softer the coating. The hardness was reflected by a hardness number: the larger the number, the softer the coating. The hardness of the coating was compared with the lubricity of the coating, and it was found that, at a needle penetration hardness in the range 20-30, lubricity was maximized for a coating of 200.+-.25 mg/ft..sup.2 (2.16.+-.0.27 g/m.sup.2).